I want to find all packages installed from the testing archive, so I try:
As it turns out, this returns every package P that is installed and is included in testing, regardless of whether the installed package actually comes from testing. E.g. bash will appear in the result list even if the installed version comes from stable, because (some other version of) bash is also in testing.

Another example: find all packages with a version that depends on two (or more) given packages, say libcairo2 and libqtcore4. Running
returns vlc among the results, although vlc/stable doesn't depend on libqtcore4 and vlc/testing doesn't depend on libcairo2 (and sources.list contains only stable and testing).

Is this possible with aptitude (and how) ?

Ok,you'll need to install apt-show-versions,so just simply:

Followed by:

For the second question you could try:

I take it you know mixing Testing and Stable is not advised?.

Well I was more interested for a general way to do SQL-like joins with aptitude rather than ad-hoc ways to answer these two specific examples (though that was handy too, thanks).

I do but it's for my laptop, not some mission critical server; for some reason I have issues with v2.6.3x kernels and Xorg 7.4 so I am running Mepis for now and handpick a few packages (+dependencies) from Testing if the Stable versions don't quite cut it.